Locations

Blog Stats

Archive for the ‘PollingDuplex’ Category

Introduction

This post attempts to highlight some of the intricacies involved in connecting a WPF Client to the existing Silverlight 2 Polling Duplex demo previously published on this blog. To recap, the demo consists of a WCF service designed to push Stock data to previously connected Silverlight 2 clients over Http. In a similar scenario to a chat application, the Silverlight 2 clients can also send ‘Note’ messages back to the server and have them propagated to other connected clients.

The aim of this post is to make the demo capable of also supporting connections from WPF clients. Success is defined by enabling WPF clients to connect to the same WCF service, receive the same Stock updates at the same time as the Silverlight 2 clients and having Notes synchronized across all connected clients regardless of their underlying technology. Below is a link to a screenshot of the final version of this demo running, shown is the Console Application hosting the WCF service with two Silverlight clients and one WPF client connected:

PollingDuplexHttpBinding and WPF clients

Up until this point the demo application has solely supported pushing data to Silverlight 2 clients, defining a PollingDuplexHttpBinding endpoint on the server side and creating a channel factory as part of the Silverlight 2 client as shown below:

However, attempting to use PollingDuplexHttpBinding for this purpose in a WPF application (as in the code above) currently results in a NotSupportedException exception being raised with a full explanation and even some architectural advice:

PollingDuplexHttpBinding cannot currently be used in non-Silverlight clients without running into this exception. In order to extend the demo to support pushing Stock data to a WPF client through Http the guidance from the above exception is to consider WSDualHttpBinding. This binding predates Silverlight and is the closest alternative for achieving equivalent results to PollingDuplexHttpBinding in a non-Silverlight client application. A look at the BindingElement objects each binding encapsulates reveals WSDualHttpBinding is actually a much more mature binding than PollingDuplexHttpBinding:

WSDualHttpBinding

PollingDuplexHttpBinding

TransactionFlowBindingElement

PollingDuplexBindingElement

ReliableSessionBindingElement

TextMessageEncodingBindingElement

SymmetricSecurityBindingElement

HttpTransportBindingElement

CompositeDuplexBindingElement

OneWayBindingElement

TextMessageEncodingBindingElement

HttpTransportBindingElement

The WSDualHttpBinding Endpoint

The outcome of acting on the aforementioned advice in the demo application is the addition of a new endpoint to the WCF service exposed from the StockServer Console Application. This could easily be specified entirely in a configuration file (see this post for how to configure the Silverlight policy and duplex endpoints) but in the download is achieved in code as shown below:

Although the WSDualHttpBinding endpoint is registered using a different URI, the endpoint is simply just another entry point into the same WCF service called by the existing Silverlight 2 clients using PollingDuplexHttpBinding. Before you can push data from a WCF duplex service to a client, the client must initiate the session by calling the service. The Register method serves this purpose in the demo application and the only change to the code in the WCF service itself (although there are no method signature changes) is in this method as shown below:

The change is minor but essential: extracting the MessageVersion from the initial message sent by the client and passing it to the constructor of the StockClient object along with a reference to the channel back to that client. The variety of MessageVersion used by each of the bindings in the demo application is as follows:

Binding

MessageVersion

WSDualHttpBinding

MessageVersion.Soap12WSAddressing10

PollingDuplexHttpBinding

MessageVersion.Soap11

Messages sent between WSDualHttpBinding endpoints use the SOAP 1.2 protocol; between PollingDuplexHttpBinding endpoints the SOAP 1.1 protocol is employed. There is a clue in the name of MessageVersion. Soap12WSAddressing10 that WSDualHttpBinding also requires addressing headers to support reliable messaging sessions via ReliableSessionBindingElement. In the case of these two bindings, attempting to use a mismatch of MessageVersion and binding, for example MessageVersion.Soap11 for messages sent to a WSDualHttpBinding endpoint, results in an InvalidOperationException as shown below:

Storing the message version along with the channel reference in an instance of the StockClient class (see code download) ensures the reply messages are pushed back to the client in the format the relevant binding expects. The rest of the WCF service remains the same: a new instance of StockService is created per session regardless of the endpoint used. Each session registers it’s interest in the DeltaReceived event of the singleton StockGenerator class. This event is raised approx every 250 milliseconds and each session’s handler (OnDeltaReceived) sends the same generated delta to its respective client via the stored channel reference using the appropriate message protocol.

The WPF Client Application

In a good advertisement for how nearly all of what you write in Silverlight 2 can be used without modification in WPF, the code in the WPF client application is extremely similar to that contained in the Silverlight 2 client project. The markup in StockWindow.xaml and code in StockWindow.xaml.cs is virtually identical to Page.xaml and Page.xaml.cs in the Silverlight 2 project.

One difference between the WPF and Silverlight client code is the WPF DataGrid which is from the October 2008 Release of the WPF Toolkit available from codeplex. Another difference already highlighted is that PollingDuplexHttpBinding cannot be used in a non-Silverlight client application. In its place we use WSDualHttpBinding on the WPF client side also and due to this binding’s maturity the code in StockTicker.cs is less verbose as shown below:

using System;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Channels;
using System.Windows.Threading;
using Common;
namespace WpfStockClient
{
public sealed class StockTicker : IStockClient
{
// Reference to layout instance that created the StockTicker
private readonly Dispatcher owner = null;
// Serializes instances of the Stock type before they are sent on the wire
private readonly DataContractSerializer stockSerializer = new DataContractSerializer(typeof(Stock));
// Proxy for communication to WCF service
private IStockService proxy;
// List of stocks designed to be bound to a UI control
private readonly StockList stockList = new StockList();
public StockList StockList
{
get { return stockList; }
}
public StockTicker(Dispatcher owner)
{
if (owner == null)
{
throw new ArgumentNullException("owner");
}
this.owner = owner;
}
public void SubscribeDeltas()
{
EndpointAddress endPoint = new EndpointAddress("http://localhost:10201/WpfStockService");
proxy = new DuplexChannelFactory<IStockService>(this,
new WSDualHttpBinding(WSDualHttpSecurityMode.None),
endPoint).CreateChannel();
proxy.Register(Message.CreateMessage(MessageVersion.Soap12WSAddressing10, "StockClient/IStockService/Register"));
}
public void Receive(Message message)
{
Stock stock = message.GetBody<Stock>();
// Queue a call to UpdateStockList on the Dispatcher of the thread that created this instance
Action<Stock> action = UpdateStockList;
owner.BeginInvoke(action, stock);
}
public void Sync(Stock stock)
{
// Create a message with the appropriate SOAPAction and asynchronously send it via the proxy with the serialized Stock as the body of the envelope
Message message = Message.CreateMessage(MessageVersion.Soap12WSAddressing10, "StockClient/IStockService/Sync", stock, stockSerializer);
proxy.Sync(message);
}
private void UpdateStockList(Stock delta)
{
// NOTE : CheckAccess is there but intellisense doesn't see it because of the [EditorBrowsable(EditorBrowsableState.Never)] attribute
if (!owner.CheckAccess())
{
throw new InvalidOperationException("The calling thread cannot access this method because a different thread owns it.");
}
// Check if this Stock is already in the collection
Stock existing = stockList.FirstOrDefault(s => s.Symbol == delta.Symbol);
if (existing == default(Stock))
{
// This delta is a new Stock
stockList.Add(new StockHighlight(delta));
}
else
{
// This delta is an existing Stock
existing.Ask = delta.Ask;
existing.Bid = delta.Bid;
if (!String.IsNullOrEmpty(delta.Notes))
{
existing.Notes = delta.Notes;
}
}
}
}
}

There are many similarities between the above code and the Silverlight 2 version; the changes are all centred on the body of the SubscribeDeltas method. This is where the client instance of the WSDualHttpBinding class is instantiated and used to establish a composite duplex channel (two one-way channels) with the new endpoint at the address exposed in the WCF service. The first parameter passed to the DuplexChannelFactory<IStockService> constructor specifies ‘this’ as the instance of the IStockService interface to receive the pushed messages from the WCF service in the Receive method. After the channel is created the initial message is sent using the correct MessageVersion as described earlier.

Linked Common Files

Those class files that are identical across both client projects are added to the Common Solution folder and are referenced using links from the respective projects where they are used:

The highlighted classes are written in one place and as such are assured of being the same structure throughout the solution (and where relevant on both sides of the wire as a consequence). As the files are linked, the code they contain will be compiled into IL that targets the respective version of the CLR implied by the type of project the classes are linked from. For the WPF project the linked classes will be compiled into an assembly that targets the desktop CLR, for the Silverlight project the linked classes will be compiled into an assembly that targets the more compact CoreCLR. Linking common files in this manner is one solution to the problem normally solved by using a class library when a common CLR is targeted by all projects.

Animating the Stock Price changes

Illustrating changes to Stock price instances displayed in the DataGrid by creating the illusion that the DataGrid cell background is changing color takes less code in WPF than in Silverlight. The code for this animation for both WPF and Silverlight 2 is contained in the StockHighlight class. The entire contents of the StockHighlight.cs file is shown below, the code between the #if and #endif directives is compiled only if the SILVERLIGHT symbol is defined as it is in the Build tab of project properties on all default Silverlight 2 projects:

The targets of the animation are the two public SolidColorBrush properties, AskHighlight and BidHighlight. The Color dependency property of these brushes is animated using color key frame animation from green for a positive change or red for a negative change. The animation for both WPF and Silverlight begins with a call to the BeginHighlight method when the corresponding Ask or Bid price property changes. The SILVERLIGHT debug symbol makes it clear to see the extra Storyboard objects and extra initialization required in the InitializeAnimation method to achieve the desired effect in Silverlight 2. You can see the references to these AskHighlight and BidHighlight properties in the WPF XAML below, also notice the xmlns:data for the DataGrid from the WPF Toolkit:

Surprisingly this is not supported by default in the release version of Silverlight 2 due to the absence of the required configuration-related classes needed to expose the Polling Duplex binding and binding element to the WCF configuration system. However, thanks to the great extensibility model of WCF it is possible to enable this scenario and bridge the gap by registering a custom binding extension.

The aim is to render the StockServiceHost class (defined in code above) redundant and instead replace it’s functionality entirely in a configuration file, in this case App.config. As you can see in the InitializeRuntime method the StockServiceHost class actually registers two endpoints: in addition to the PollingDuplexHttpBinding endpoint there is the WebHttpBinding endpoint to account for the Silverlight 2 cross-domain policy requirements. It would defeat the aim to succeed in defining one of these endpoints in the configuration file without the other, so the measure of success is to be able to define both in App.config. How this aim can be achieved is shown below:

Notice under the <bindingExtensions> element that the pollingDuplexHttpBinding item relies on the following two configuration-related classes (demonstrated in use as part of the sample application or available separately for download above) in the PollingDuplexConfig.cs file:

To configure Polling Duplex in Silverlight 2 solely via the .config file the PollingDuplexHttpBindingCollectionElement class including the relevant namespace and assembly, need to be referenced in the type attribute of the pollingDuplexHttpBinding item under the <bindingExtensions> element as shown in the configuration markup above.

As this is a standard way to create and register a custom binding extension, once the configuration classes and markup are in place as shown and the project successfully builds, you can edit the binding in the same way as the built-in WCF bindings by opening the produced .config file (StockServer.exe.config in this case) in SvcConfigEditor.exe while the service is running:

PollTimeout setting on the server side (PollingDuplexHttpBinding and PollingDuplexBindingElement) has been renamed to ServerPollTimeout

PollTimeout setting on the PollingDuplexBindingElement (client side) has been renamed to ClientPollTimeout.

PollTimeout setting on the PollingDuplexHttpBinding (client side) has been cut. In most scenarios, it should not be necessary to change this. If a change is necessary, it can be achieved through the ClientPollTimeout on the PollingDuplexBindingElement.

Client-side support has been cut from the non-Silverlight (server-side) polling duplex assembly (i.e. BuildChannelFactory will throw a NotSupportedException). That is, in RTM, the client side for polling duplex must be Silverlight (and the server side must be the regular .NET Framework, but this restriction was already in place in Beta2).

Default timeouts have been changed for the Duplex channel. For most common scenarios, the new out-of-the-box defaults should be appropriate and there is no need to change them.

An error (404) on a polling request will cause the duplex channel to fault.

Various invalid messages used to be ignored by the Duplex stack but will now be rejected.

If any HTTP error (404,500,…) is encountered during a SOAP call, a CommunicationException is now thrown instead of a ProtocolException.

Earlier this month I put together a series of posts representing somewhat of a deep-dive into the WCF Polling Duplex support in Silverlight 2 Beta 2. The series is in three parts covering Architecture, The Server and The Client and will still be largely relevant. The source code for a Beta 2 sample application discussed across parts two and three is available for download.

I will be on vacation over the next week-and-a-bit and won’t be able to respond to emails/queries immediately but I do fully intend to update the polling duplex sample application to account for these breaking changes when I return. Applications built with Beta 2 will not run on final runtime builds so I will also update the other samples available on this blog including the pie/doughnut chart and Sockets samples in the near future.

This post takes a brief look at the options for capturing localhost HTTP traffic in the superb Fiddler2 tool but in particular demonstrates how this can be achieved using the less-renowned ipv4.fiddler keyword in a Silverlight 2 polling duplex debugging session.

HTTP traffic sent to or originating from http://localhost or http://127.0.0.1 by Internet Explorer and the .NET Framework is not captured in Fiddler2 by default because the requests and responses do not pass through the WinInet proxy that it registers. A few simple workarounds do exist to commonly facilitate the capture of this traffic (without customizing Fiddler’s rules) however:

Using 127.0.0.1 with a dot suffix

Using the Machine Name

Using ipv4.fiddler

The rest of this post shows the results of using each of these methods in an attempt to achieve a complete trace of all HTTP traffic being sent and received between my sample Silverlight 2 Beta 2 polling duplex client and server application, the results were quite interesting. The sample application consists of three projects:

The Silverlight 2 Beta 2 client polling duplex application

The Web Application hosting the Silverlight client

The WCF polling duplex service hosted in a Console Application

The server manually handles the Silverlight client access policy requirements and messages are sent in both directions by both parties. Capturing all the traffic between the client and server in Fiddler2 requires the base http://localhost url to be manipulated at design time according to the appropriate workaround in each of the three projects. Let’s take a look at the process using each workaround in turn and the ensuing results:

1. Using 127.0.0.1 with a dot suffix

I first read about this technique here and it does work in a number of more common scenarios, let’s see how it fares in this scenario:

A. Silverlight Client

Firstly the manipulation of the default http://localhost url used to connect to the server duplex service from the Silverlight 2 client project:

// From this
EndpointAddress endPoint = new EndpointAddress("http://localhost:10201/StockService");
// To this - Note the period after localhost
EndpointAddress endPoint = new EndpointAddress(http://127.0.0.1.:10201/StockService);

B. Hosting Web Application

Secondly the same change to the Start URL for the web application that hosts the Silverlight 2 client:

C. WCF Service Console Application

Lastly a change to the base address of the WCF polling duplex service in the server’s configuration file:

D. Results in Fiddler

The screenshot above shows the results of our efforts using this workaround. The session gets as far as the GET request for the built-in clientaccesspolicy.xml file. Due to our manually added suffix a HTTP 400 Bad Request – Invalid Hostname response is returned. This HTTP response is eventually translated into the following exception in the Silverlight client:

2. Using the Machine Name

This is a technique listed in the relevant help section on the Fiddler2 website, lets see what results this workaround yields:

A. Silverlight Client

Again we start with the manipulation of the default http://localhost url used to connect to the server duplex service from the Silverlight 2 client project:

// From this
EndpointAddress endPoint = new EndpointAddress("http://localhost:10201/StockService");
// To this (i.e. Environment.MachineName)
EndpointAddress endPoint = new EndpointAddress(http://vgnar21s:10201/StockService);

B. Hosting Web Application

Again, the same change to the Start URL for the web application that hosts the Silverlight 2 client:

C. WCF Service Console Application

Lastly a change to the base address of the WCF polling duplex service in the server’s configuration file:

D. Results in Fiddler

The screenshot above shows the results of our efforts using this workaround. The session does not get past the request for the aspx page that hosts the Silverlight application and a HTTP 502 Connection Failed response is returned. This HTTP response message eventually appears as the following error in Internet Explorer:

3. Use ipv4.fiddler keyword

At this stage the two previous workarounds have both proven unsuccessful, let’s try the same process this time with another technique listed in the relevant help section on the Fiddler2 website – use of the ipv4.fiddler keyword. The ipv4.fiddler keyword requires Fiddler v2.1.8 or later to be running, I should also mention the ipv6.fiddler keyword (not covered in this post) for IPV6 debugging.

A. Silverlight Client

Firstly the manipulation of the default http://localhost url used to connect to the server duplex service in the Silverlight 2 client project:

// From this
EndpointAddress endPoint = new EndpointAddress("http://localhost:10201/StockService");
// To this (Fiddler keyword)
EndpointAddress endPoint = new EndpointAddress(http://ipv4.fiddler:10201/StockService);

B. Hosting Web Application

Secondly the same change to the Start URL for the web application that hosts the Silverlight 2 client:

C. WCF Service Console Application

Lastly a change to the base address of the WCF polling duplex service in the server’s configuration file:

D. Results in Fiddler

Success – this workaround accomplishes exactly what we set out to achieve, the screenshot above shows the capture of all HTTP traffic going between the client and server. It shows everything from the initial request for the hosting aspx page, downloading the .xap file, the request and custom self-hosted response for the clientaccesspolicy.xml file and finally the actual polling duplex activity on the wire. Note the content of the User-defined column, this is as a result of turning on ‘Show Time-to-Last-Byte’ and ‘Show Response Timestamp’ under the Rules > Performance menu in Fiddler2.

For a deep-dive into exactly what these polling duplex requests and responses contain, please see the ‘Anatomy of a Bi-Directional Message Exchange’ heading in Part 1 – Architecture of my series of posts on polling duplex in Silverlight 2.

Introduction

This is the third and final post in my current series on the support for duplex communication with a WCF service added to Silverlight 2 Beta 2. I should point out that this technology in its current form has been deemed by Microsoft as not yet fit for production applications and is therefore currently for evaluation only. In this post I introduce the client part of a sample application available for download (the download includes the server discussed in the previous post also) that uses the bi-directional communication discussed in part one. The sample application as a whole represents a simple client and server application that illustrates polling duplex communication in both directions. The client displays stock prices in a grid and animates updates received from the server. The user of the client application can also enter notes against a stock and have those notes propagate to all other connected clients via the server using polling duplex communication in the other direction. Here is a screenshot of the sample application running with the server and two connected clients, the same stock price updates are being displayed by both clients and the notes entered in Internet Explorer have been pushed to another instance of the client running in Firefox:

The Client

The layout of the Visual Studio 2008 solution is as follows:

There are three projects in the PollingDuplexDemo solution, the first is the Silverlight Application project StockClient that references the client-side version of System.ServiceModel.PollingDuplex.dll. The .xap file produced by building this project is referenced by the Silverlight ASP.NET server control in the StockClientTestPage.aspx file as part of the hosting StockClientHost Web Application project. The StockServer project is a Console Application project that self-hosts the polling duplex WCF service. As you can see from the image there are multiple startup projects, the first to start is the StockServer project along with the StockClientHost project shortly after.

Page.xaml and Page.xaml.cs

The markup in Page.xaml (see code download) defines the StocksGrid Silverlight DataGrid along with the rest of the UI elements, the code in Page.xaml.cs adds some initialisation code to the default constructor and handlers for some UI events. When the Silverlight Application loads and assigns an instance of the Page class defined across these files to the RootVisual property of the Application class, the constructor shown below is executed:

The StockTicker Class

It’s apparent from the code above that the instance of the StockTicker class is central to everything happening in the client application and indeed it’s the code in this class that implements the client-side PollingDuplex support:

namespace StockClient
{
public sealed class StockTicker
{
// Reference to layout instance that created the StockTicker
private readonly Dispatcher owner = null;
// Object used to let ThreadPool know to stop waiting and proceed
private readonly AutoResetEvent waitObject = new AutoResetEvent(false);
// Asynchronously begins an open operation on an ICommunicationObject with code to call EndOpen when it completes
private static readonly Action<ICommunicationObject> Open =
ico => ico.BeginOpen(iar => ico.EndOpen(iar), ico);
// Asynchronously begins a send operation on an IDuplexSessionChannel with code to call EndSend when it completes
private static readonly Action<IDuplexSessionChannel, Message> Send =
(idc, msg) => idc.BeginSend(msg, iar => idc.EndSend(iar), idc);
// Asynchronously begins a receive operation on an IDuplexSessionChannel with code to call an Action<Message> when it completes
private static readonly Action<IDuplexSessionChannel, Action<Message>> Receive =
(idc, act) => idc.BeginReceive(iar => act(idc.EndReceive(iar)), idc);
// Serializes instances of the Stock type before they are sent on the wire
private readonly DataContractSerializer stockSerializer = new DataContractSerializer(typeof(Stock));
// Channel for communication to WCF service
private IDuplexSessionChannel channel;
// List of stocks designed to be bound to a UI control
private readonly StockList stockList = new StockList();
public StockList StockList
{
get { return stockList; }
}
public StockTicker(Dispatcher owner)
{
if (owner == null)
{
throw new ArgumentNullException("owner");
}
this.owner = owner;
}
public void SubscribeDeltas()
{
// Create a channel factory capable of producing a channel of type IDuplexSessionChannel
IChannelFactory<IDuplexSessionChannel> factory =
new PollingDuplexHttpBinding().BuildChannelFactory<IDuplexSessionChannel>();
Open(factory);
// Address of the polling duplex server and creation of the channel to that endpoint
EndpointAddress endPoint = new EndpointAddress("http://localhost:10201/StockService");
channel = factory.CreateChannel(endPoint);
Open(channel);
// Create a message with the appropriate SOAPAction and asynchronously send it on the channel
Message message =
Message.CreateMessage(MessageVersion.Soap11, "Silverlight/IStockService/Register");
Send(channel, message);
// Use the thread pool to start only one asynchronous request to Receive messages from the server
// Only start another asynchronous request when a signal is received that the first thread pool thread has received something
ThreadPool.RegisterWaitForSingleObject(waitObject,
delegate { Receive(channel, CompleteReceive); },
null,
Timeout.Infinite,
false);
waitObject.Set();
}
public void Sync(Stock stock)
{
// Create a message with the appropriate SOAPAction and asynchronously send it on the channel with the serialized Stock as the body of the envelope
Message message =
Message.CreateMessage(MessageVersion.Soap11, "Silverlight/IStockService/Sync", stock, stockSerializer);
Send(channel, message);
}
private void CompleteReceive(Message message)
{
// Deserialize the body of the SOAP message into a Stock object
Stock stock = message.GetBody<Stock>();
// Queue a call to UpdateStockList on the Dispatcher of the thread that created this instance
Action<Stock> action = UpdateStockList;
owner.BeginInvoke(action, stock);
// Signal the thread pool to start another single asynchronous request to Receive messages from the server
waitObject.Set();
}
private void UpdateStockList(Stock delta)
{
// NOTE : CheckAccess is there but intellisense doesn't see it because of the [EditorBrowsable(EditorBrowsableState.Never)] attribute
if (!owner.CheckAccess())
{
throw new InvalidOperationException("The calling thread cannot access this method because a different thread owns it.");
}
// Check if this Stock is already in the collection
Stock existing = stockList.FirstOrDefault(s => s.Symbol == delta.Symbol);
if (existing == default(Stock))
{
// This delta is a new Stock
stockList.Add(new StockHighlight(delta));
}
else
{
// This delta is an existing Stock
existing.Ask = delta.Ask;
existing.Bid = delta.Bid;
if (!String.IsNullOrEmpty(delta.Notes))
{
existing.Notes = delta.Notes;
}
}
}
}
}

In order to create an instance of the StockTicker class a reference to an instance of the System.Threading.Dispatcher class is required. In the initialisation of the StockTicker instance in the Page class constructor the Dispatcher property inherited all the way from System.Windows.DependencyObject is used. This provides StockTicker with a queue to add tasks to for execution on the UI thread. It is essential to update collections that may be bound to UI controls (such as the StockList property) from the UI thread to avoid an invalid cross-thread access (UnauthorizedAccessException) exception; this restriction also applies to the animation of individual stock prices using Storyboards managed by the StockHighlight class. The stored Dispatcher reference is later used to queue calls to UpdateStockList to be executed on the UI thread from the CompleteReceive method and to check the correct thread has called UpdateStockList before updating the collection or the Stock properties that cause animation Storyboards to begin.

After the instance of StockTicker has been created in the constructor of the Page class the SubscribeDeltas method is called. This method performs all of the client setup necessary to begin a polling duplex session with the WCF service. The method body begins by creating an instance of the client-side version of PollingDuplexHttpBinding (see part one) and extracting an instance of a channel factory of type IDuplexSessionChannel from that binding. The factory is then opened asynchronously and the channel is created, the client then initiates communication by asynchronously opening the channel to the WCF service specified in EndpointAddress. The method progresses to create a SOAP 1.1 Message instance specifying the One Way Register method discussed in the previous post in the SOAPAction; the instance of this message is then sent asynchronously along the channel to the server. Finally the method uses an AutoResetEvent to signal a ThreadPool thread to begin one asynchronous request to receive messages pushed from the server. When a message is eventually received the CompleteReceive callback method is executed to queue a call back to the UI thread with the received Stock object before signaling the wait handle; this signal in turn prompts the thread pool to release another single asynchronous request to receive another message.

The Sync method creates a new SOAP 1.1 Message with a different SOAPAction and with the Stock parameter (containing the Notes entered by the user) as the body of the envelope serialised via the DataContractSerializer. The method then uses the same channel opened in SubscribeDeltas to asynchronously send the message to the server for propagation to all connected clients. The code for the WCF service methods being called here is discussed in part two, the make up of these asynchronous calls and their responses on the wire is discussed under the ‘Anatomy Of A Message Exchange’ heading In part one.

The UpdateStockList method checks the current thread as previously described and then searches the StockList collection for the current Stock object comparing Symbol properties. A new instance of the StockHighlight class is created to add to StockList if the received stock does not exist in the collection. As StockList is an ObservableCollection this insert will notify the DataGrid that it’s ItemsSource has changed and to update itself to honour the addition in the UI. If the stock already exists then only the relevant price and notes properties on the existing StockHighlight instance are updated, triggering the relevant animations. These updates in turn cause the PropertyChanged event from INotifyPropertyChanged implemented by the Stock class (see code download) to fire again causing the DataGrid to update the UI.

The StockHighlight Class

This class extends the basic Stock class overriding the Ask and Bid properties and exposing two public properties of type Brush:

Each cell in the Bid and Ask columns of the DataGrid declared in Page.xaml contains a StackPanel whose Background property is bound to the respective Brush property. This way each cell is able to play it’s own unique animation specific to the Stock displayed. These Brush properties need to be part of each individual Stock object rather than global resources so the whole column does not change colour when one stock changes price. The InitializeAnimation method associates animations of type ColorAnimationUsingKeyFrames with each of these Brushes for both positive and negative changes. The appropriate Storyboard to play is chosen in the Bid and Ask property setters based on the new value.

The most salient parts of the client code are now covered, download the solution, press F5 to start the server and one client automatically, you can then browse to StockClientTestPage.aspx using the same address from multiple clients in separate browsers once the service is running.

In the future it looks likely that the client side Silverlight 2 Polling Duplex API will be simplified to enable a less verbose method of connecting to and receiving pushed messages from a WCF service. This post talks more about the possibility of a DuplexReceiver<T> class for simple client scenarios in the future; it’s always prudent to know what such a class will be doing for you behind the scenes.

Further Reading

A few links on polling duplex support in Silverlight 2 Beta 2 you might find useful:

Introduction

This is the first in a series of posts on the support for duplex communication with a WCF service added to Silverlight 2 Beta 2. I should point out that this technology in its current form has been deemed by Microsoft as not yet fit for production applications and is therefore currently for evaluation only. In this post I take a look at the general architecture of the technology and it’s approach to solving the problem of bi-directional communication between a server and a browser. Parts two and three introduce the code for a client and server sample application that demonstrates the technology in practice.

The Assemblies

Support for bi-directional communication between a WCF Service and a Silverlight 2 Client over HTTP (port 80 by default) was introduced as part of the Silverlight 2 Beta 2 SDK. This is different to the Sockets infrastructure also supported by Silverlight 2 that requires a custom port within a specific range to be opened. Two assemblies both named System.ServiceModel.PollingDuplex.dll provide this duplex support, one for the client, one for the server. It’s clear to see which is which from the location each assembly is extracted to by the SDK installer:

Location aside, it’s possible to discern between the two based on the public key token of mscorlib.dll the assembly was built against (thanks to this post). The client assembly is built against the Silverlight version, the server assembly against the .NET Framework version:

Both assemblies host types in two main namespaces familiar to WCF developers: System.ServiceModel and System.ServiceModel.Channels. The content of the two assemblies in terms of types in these namespaces is similar but not identical. The server side assembly has around nine additional internal types (PartialTrustHelpers, ContextQueue, IOThreadTimer, PollingDuplexChannelListener, RequestContextData, SafeNativeMethods, ServicePollingDuplexDispatcher, ServicePollingDuplexSessionChannel and UnsafeNativeMethods) with names that suggest they provide functionality outside of what can be achieved in the client sandbox. The server assembly is therefore slightly larger than the client (101KB vs. 81KB) but from an API perspective they are identical in that each exposes only two public types: System.ServiceModel.PollingDuplexHttpBinding and System.ServiceModel.Channels.PollingDuplexBindingElement. A little more exploration into these two types reveals that an instance of the latter is encapsulated by the former. By this simple process of elimination it’s clear to see that creating an instance of the PollingDuplexHttpBinding type from both the client and server assemblies is the starting point to enabling WCF duplex communication in Silverlight 2 Beta 2.

The Binding

There are many different types of WCF Bindings, you can think of each one as a unique package of decisions with regard to the transport, message encoding, security and protocol details that will be employed to transfer a communication on the wire between a client and server or vice versa. With these decisions encapsulated by the binding, enabling congruent communication between the client and server is simply a matter of instantiating the same binding type on both sides. The PollingDuplexHttpBinding binding inherits from the standard WCF BasicHttpBinding class and specializes it to produce a new WCF binding specifically designed for communication with a Silverlight 2 client application. The decisions packaged in the binding by default are to use HTTP as the transport, UTF-8 text as the message encoding, BasicHttpSecurityMode.None as the security setting (configurable in the constructor) and in addition to HTTP, Net Duplex and WS-Make Connection (WS-MC) as the protocols. These protocols in turn rely on the XML and SOAP specifications, the Net Duplex Protocol is implemented to establish the Silverlight 2 client session with the server and WS-MC is used to establish a back channel using polling so that server initiated messages can be delivered to the client. These protocols are specifically designed to be used in place of more traditional duplex protocols like TCP and cater for the fact that a Silverlight application running in a browser cannot accept new incoming connections unless a custom port is opened and agreed upon. Another way to look at this is that without opening a custom port a Silverlight client application is not addressable by protocols such as TCP by default because the browser environment does not allow the creation of a listener. Consequently, with polling duplex communication the server must be addressable and the Silverlight application must initiate the connection to it. In order to create the illusion of duplex communication (where either party can initiate a message exchange) in such a scenario, Microsoft have chosen to have the client poll the server for return messages after initiating the connection.

Polling and Timeouts

Once connected and until a fault or timeout occurs the client asynchronously polls the server more or less constantly give or take a few milliseconds between poll timeouts. A poll returns when either there is a message to convey to the client or a poll-timeout occurs. The default poll-timeout is currently around 60,000 milliseconds (60 seconds rather than 90 as you may have read elsewhere) but this can be altered by setting the PollTimeout property of an instance of PollingDuplexHttpBinding which will be forwarded to the identically named property of the binding’s encapsulated instance of PollingDuplexBindingElement. When a PollTimeout occurs an exception is not raised unless either the client or server only has aborted the session during that time in which case the following exception is raised:

When the session has not been aborted and is still active, the occurrence of a PollTimeout does not raise an exception, an empty response (HTTP/1.1 200 OK with Content-Length: 0) is returned and a new poll originates on the network layer. In the few milliseconds after a poll timeout occurs and before a new poll occurs, a queue of any messages to be sent to the client is maintained on the server ready to be flushed on the next poll. There is another type of timeout that does raise an exception every time it occurs. InactivityTimeout by default is set to 10 minutes (again configurable via the binding) and so will occur after 10 poll timeouts in a sequence, that is – after a period of 10 minutes during which no bytes have been sent or received by either party. The following image shows the exception raised after an InactivityTimeout:

Anatomy of a Bi-Directional Message Exchange

In order to illustrate what I’ve covered so far and progress further a familiar context needs to be established in the form of a sample application. The second and third posts in this series introduce the server and client parts of the sample application respectively and I encourage you to download the Visual Studio 2008 Solution, familiarize yourself with how it works and have a look at the code and comments. The solution represents a simple client and server application that illustrates polling duplex communication in both directions. The client displays stock prices in a grid and animates updates received from the server. The user of the client application can also enter notes against a stock and have those notes propagate to all other connected clients via the server using polling duplex communication in the other direction. Here is a screenshot of the sample application running with the server and two connected clients, the same stock price updates are being displayed by both clients and the notes entered in Internet Explorer have been pushed to another instance of the client running in Firefox:

An excerpt of the most significant bi-directional conversation between one client and server assuming no communication problems would include the following events:

This is a HTTP POST of the Net Duplex request from the client to the server to establish a unique session. Note the presence of Connection: Keep-Alive and the SOAPAction in the HTTP request specifying the WCF service method to call (see part two for the server code). The Net Duplex information is present as part of the SOAP header and includes two separate GUID identifiers in the Address and Session elements. The Address element appears to specify an identifier for use by WS-MC (note the URI part of the identifier is the WS-MC anonymous URIalthough anonymous is spelt incorrectly) and the SessionId element appears to uniquely identify the current Net Duplex session; the SOAP body is empty.

The above is a second HTTP POST to the server from the client a few milliseconds after the initial Net Duplex request. This time the SOAPAction identifies it as a WS-MC MakeConnection request. The SOAP envelope has no header but the body contains the same URI and GIUD identifier established in the Address element of the previous Net Duplex request.

The above is the first real response sent back along the HTTP response channel or "back-channel" (a channel capable of carrying a SOAP message, without initiating a new connection) in reaction to the client’s connection request and polling. This time the server has queued a message destined for this unique session’s client in the form of a serialized stock object with the symbol AAHH. The content-length is therefore greater than zero this time as the response consists of a SOAP envelope with the Net Duplex information in the header and the serialized Stock object in the body. See the server code in part two for how the Stock objects are serialized using the DataContractSerializer.

This time we see a similar SOAP envelope to the previous response except travelling in the opposite direction. The SOAPAction in the HTTP POST request is specifying that the Sync method of the WCF service should be called (again see part two for the server code). The SOAP envelope again contains the identifying Net Duplex information in the header and the body contains a single serialized stock instance, this time with some notes entered by the user.

On receipt of the notes post from the client, if there are no messages pending the server sends back an empty response as shown earlier. A few milliseconds later when a client polls again or if there are other clients polling as shown previously, the received update is echoed back in the response above. To see the entire client and server code involved in generating these transmissions, download the sample Visual Studio 2008 solution attached to parts two or three.

Comet

Like all good problems, server to browser communication has prompted many slightly different solution implementations over the years. The term Comet was first defined here (the technologies covered by the umbrella existed long before) as an umbrella term used to discuss these different implementations as a group when necessary (although it seems at one point it was nearly used as the codename for ASP.NET AJAX instead of Atlas). More commonly the term is used in context with browser-native technologies such as JavaScript but I have seen it used to include proprietary plugins such as Silverlight. The browser-native implementations apply methods of maintaining long lived connections between a browser and a server commonly using streaming or long-polling to enable partial page updates from JavaScript based on server initiated events. If done correctly, this can result in near real time updates to a web-page without it ever explicitly requesting those updates. Just some examples of applications that already use this kind of technology include:

Historically, browser-native Comet implementations have had to overcome several obstacles often based around the fact that the underlying technologies were not specifically designed to work in the way they were being manipulated. Examples include browser compatibility, firewalls terminating long running connections and problems with incorrectly configured proxies. Another obstacle is that keeping a long-lived connection open to a particular server uses up one of the simultaneous connections allowed by the HTTP 1.1 specification (have a look at the sixth paragraph under heading 8.1.4 here). This can lead to browser usability problems but is often overcome by using a sub-domain on the same server as the initiator of the update events only. It is very interesting to note that there are some changes around this restriction due in Internet Explorer 8 for broadband connections. From a Silverlight perspective, as the implementation runs inside a plug-in the cross-browser compatibility obstacle is alleviated as long as the user has the plug-in installed. These obstacles and their many different solutions by the different implementations of Comet in use today imply that no one solution is yet perfect across all browsers in all scenarios. Work is underway to attempt to do something about this by standardising native support for server-sent events, event streaming and Web sockets in the HTML 5 specification, the latest Editor’s Draft at the time of writing is as up-to-date as a few days ago on 29 August 2008. Native browser support for such communication in the future prompts musings on whether Microsoft could add not only polling but streaming duplex support to ASP.NET applications in the future using SOAP/JSON, the ASP.NET AJAX client framework, and an ASP.NET server control or WCF; and whether the messages would be compatible with this Silverlight framework on the wire and over the HTML bridge.